1. Field of the Invention
The present invention relates to an electrophotographic photosensitive member, and, more particularly, to an electrophotographic photosensitive member having a wide sensitivity region over the visible light to infrared regions.
2. Related Background Art
Hitherto known are electrophotographic photosensitive members in which inorganic photoconductive material such as selenium, cadmium sulfide and zinc oxide are utilized as photosensitive components.
On the other hand, since discovery of the fact that particular organic compounds show photoconductivity, a great number of organic photoconductive material have been developed. For example, known are organic photoconductive polymers such as poly-N-vinyl carbazole and polyvinyl anthracene, low molecular organic photoconductive materials such as carbazole, anthracene, pyrazolines, oxadiazoles, hydrazones and polyarylalkanes, and organic pigments or dyes such as phthalocyanine pigments, azo pigments, cyanine dyes, polycyclic quinone pigments, perylene type pigments, indigo pigments or squaric acid methine dyes.
In particular, since the organic pigments or dyes having photoconductivity can be readily synthesized as compared with inorganic materials and moreover have gained extended variations from which the compounds having the photoconductivity at any suitable wavelength region can be selected, there have been proposed a great number of photoconductive organic pigments or dyes.
For example, as disclosed in U.S Pat. No. 4,123,270, U.S. Pat. No. 4,247,614, U.S. Pat. No. 4,251,613, U.S. Pat. No. 4,251,614, U.S. Pat. No. 4,256,821, U.S. Pat. No. 4,260,672, U.S. Pat. No. 4,268,596, U.S. Pat. No. 4,278,747, U.S. Pat. No. 4,293,628, U.S. Pat. No. 4,356,243, U.S. Pat. No. 4,471,040, U.S. Pat. No. 4,582,771, etc., known are electrophotographic photosensitive members wherein disazo pigments showing photoconductivity are used as charge-generating materials in photosensitive layers functionally separated into charge generation layers and charge transport layers.
The electrophotographic photosensitive members employing such organic photoconductive materials can be produced by coating with suitable selection of binders. Accordingly, they are advantageous in that the productivity is so high that there can be provided inexpensive photosensitive members and moreover the photosensitive wavelength region can be arbitrarily controlled by selecting the organic pigments or dyes.
In particular, lamination type photosensitive members obtained by laminating a charge transport layer and a charge generation layer chiefly comprised of charge-generating materials are advantageous in sensitivity and in the increase in the residual electric potential after durability testing as compared with other single layer type photosensitive members, and have already put into practical use.
On the other hand, when controlling the photosensitive wavelength region by selecting the charge-generating materials, it is difficult to find a material (or panchromatic material) having a wide photosensitive wavelength region with a single material. For this reason, as disclosed in U.S. Pat. No. 3,992,205, U.S. Pat. No. 4,026,704, etc., it is known to mix two or more kinds of charge-generating materials having different photosensitive wavelength region.
In this instance, however, there occur other problems originating from the charge-generating materials contained in the different two or more kinds.
Since in general the charge-generating materials have no film-forming properties when used alone, the charge generation layer is formed by coating a solution prepared by dispersing the materials in a solvent and a binder resin. However, because of the respectively different dispersibility of the charge-generating materials, it is difficult to simultaneously disperse the charge-generating materials of two or more kinds.
Even when the solutions prepared by separately dispersing the materials are mixed, the agglomeration between dispersed particles of the different charge-generating materials tends to occur and good stability cannot be obtained.
If a charge generation layer is formed in this manner, its properties as an electrophotographic photosensitive member are not sufficient, bringing about the problems such that, for example, the sensitivity of a certain charge-generating material at the maximum absorption peak wavelength becomes lower than the instance where a charge generation layer has been formed by using it alone, or the dark decay and light memory are too large to obtain stable images in repeated electrophotographic processes.